The present invention relates generally to direct access storage devices, and, more specifically, to shock protection thereof.
A direct access storage device (DASD) in the exemplary form of a conventional computer hard disk drive (HDD) includes a data storage disk having thousands of concentric data tracks along which data is written or read magnetically. The disk spins during operation, and a transducer or access head flies atop the disk surface on an air bearing to prevent undesirable contact therebetween. The access head is supported atop a slider which effects the air bearing upon spinning of the disk, and the slider in turn is supported by a suitable suspension which is selectively pivoted by an actuator to position the access head at any specific one of the data tracks.
Hard disk drives for portable computer applications require higher than conventional levels of shock immunity. Operating shock of the order of 120 g/3 ms is known to cause damage to head-disk-interface (HDI). The mechanism for HDI damage during operating condition is not well defined. Thus any systematic refinement to enhance the failure threshold of HDI is yet to be developed.
Other than removing the slider from the disk surface using Load/Unload (L/UL) technology no other prior art is known in this field for avoiding the HDI damage. Since unloading the slider from the disk surface and then reloading it back again is time consuming, this is done either intermittently or during power-down. Hence, a technique for minimizing the HDI damage without compromising performance during operational mode is highly desirable.
The slider in a HDD remains mostly in an idle operating position determined by its most recent read-write, or access, operation. The actual read or write operation takes only a fraction of the operating time in a laptop application. The user data is stored in the conventional data-zone that spans the disk surface. An outer diameter landing zone is dedicated for the slider to rest during the non-operating condition. The vulnerability of this configuration is that the slider is positioned over the data-zone during all operating times, including the idle mode. Thus any operating shock is prone to damage the data if the shock level exceeds specification.
Under operating conditions, the slider flies over the disk surface by maintaining an air bearing between itself and the disk. A moderate shock applied perpendicular to the plane of the disk can cause the slider to move significantly. This motion of the slider can disrupt the air bearing and cause contact between the slider and the disk surface. Since the operational disk is spinning, any contact between the slider and the disk can result in damage to the protective overcoat on the disk surface and to the magnetic layer underneath. If the contact occurs over an area where user data is stored, the damage to the magnetic layer can result in irrecoverable data loss.
The exact mechanics leading to HDI damage under operating conditions is not yet well understood. An experiment to determine the onset of slider disk contact due to shock was conducted. In this experiment, sensitive accelerometers were mounted on the arm of a 2.5" disk drive and the output monitored while external shocks were applied. An increase in high frequency noise output of the sensor indicated contact between the slider and the disk.
The contact threshold is fairly insensitive to slider position, but sensitive to shock pulse width. From a dramatic increase in output, the contact threshold (for shock pulses with medium width) was determined to occur at about 125 g. Since hard errors were seen in drives that had been subjected to shocks of about 160 g, one can conclude that data loss occurs soon after slider-disk contact.
Experimental testing of computer systems indicate that shock levels on disk drives can exceed the operating shock threshold easily. Shock levels up to about 582 g's were seen on the drive when the system was dropped by a few inches. Since any user is likely to subject the system to such drops due to inadvertent mishandling even when the unit is operational, the probability of data loss due to HDI damage from operating shock remains high.
Accordingly, it is desirable to increase data integrity in the data storage disk subject to shock induced contact between the slider and disk surface.